Wet scrubbers are used in dust collection for a variety of reasons. One of the reasons is to help prevent fires and explosions.

Wetting Particles Using Wet Scrubber

Since wet scrubbers use water to help with dust removal their environments usually have a very high humidity. In this environment dust particles become wet and this can often prevent them from catching fire and/or exploding. Material such as wood has a much different fire danger when it is wet compared to dry.

As we see on the news, when there is a draught there are more forest fires around the country. This also applies to wet scrubbers. Spraying water on a solid can often prevent an explosion or fire. This is done by wetting the particles thereby making it harder for them to combust but also by wetting down any spark or fire that could cause an explosion.

Material Used vs. Wet Scrubber

If you are using a wet scrubber there are some thing you should keep in mind. Some material, such as alkali metals, will react with water and potentially explode. Other metals like magnesium can also cause fires and explode with water. Keep in mind that controls need to monitor the water in the wet scrubber system, because if it loses water, an explosive situation could quickly develop. 

So when looking at an explosive application, wet scrubbing can often provide dust removal for you, but not always. There are certain disadvantages of wet scrubbing that should be reviewed before determining if it is the right solution for you.

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Airlocks are required in order for dust collectors to operate correctly. When a dust collector is under positive pressure, the airlock prevents air and collected material from blowing out of the dust discharge flange. A lack of airlock will cause dust to be thrown into surrounding areas and pile up on the floor and other surfaces.

When a dust collector is under vacuum, an airlock prevents air from entering the dust collector from the dust discharge flange. If no airlock is installed, air will leak in through the dust discharge, re-entraining material from the hopper while lowering the airflow at the pickup points.

Which Type of Airlock You might Use

Every airlock will leak a certain amount of air into or out of the hopper depending on the conditions the hopper is under. Depending on your application, the amount of acceptable leakage might determine the type of airlock you use.

A knife gate has the greatest amount of leakage, because when the knife gate is open, there is no airlock, only when you close the knife gate will you achieve an airlock.

A rotary valve provides a good, steady airlock but in order for the rotor to turn, there must be a gap between the rotor and the housing. This gap allows air to constantly leak past the rotor. The rotary valve also will allow air pass if the pockets in the rotor are not fully filled with material.

The double dump valve provides one of the lowest leakage rates of an airlock. The double dump valve is similar to having two (2) knife gates in series. This creates a chamber between the two flap plates. At all times one of the flap plates is closed, so the total leakage in the double dump valve is the volume of the chamber minus the volume of the material in the chamber.

Aerodyne’s Vacu-Valve

Aerodyne also produces a Vacu-Valve, which provides low leakage. The Vacu-Valve can only be used on applications with a slight vacuum and must have material that is fine and free-flowing (doesn’t easily bridge). The Vacu-Valve has an opening that a rubber sleeve is attached to. This sleeve is pulled together by the vacuum in the hopper.

Material then begins to build up above the sleeve. As the material builds up, gravity and the weight of the material slowly trickles the material down through the sleeve and out of the system. This trickle action is dependent on many conditions, including the weight of the material, the amount of vacuum in the system, and the ease of the particles to flow through the sleeve (particle size, stickiness, etc.)

So when selecting an airlock for you dust collector, don’t forget to factor in the air leakage each airlock will allow.

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Using bags, cartridges, and filters for dust collection is one of the best ways of capturing dust particles in the airstream. The filter allows the air to pass by while preventing the particulate from passing through. The dust particles then begin to build up on the filter. This build up helps prevent smaller particles from passing through, but also makes it harder for air to pass through. Over time this dust layer will prevent the air from passing though, thereby plugging up the filters. 

To prevent plugging of the filters, they are often cleaned by blowing air through them in the opposite direction of airflow. Baghouses and cartridge collectors use compressed air which expands the filter slightly so that it drops off material. The dust then falls into the hopper below. The periodic expansion of the filters will over time cause holes to form.

Dangers in Cleaning Out Filters

For HEPA filters and other inline filters, it is not as easy to clean filters during operation. Usually the filter needs to be pulled out of service and then air is blown through the filter (in the opposite way) to remove the dust from the filter. However, using high pressure air to blow out the filters can also cause holes to develop in the filter.

When these holes develop in filters, air will rush through the holes, taking dust with them. Since the holes in the media have lower resistance than the filters, air will find it easier to go through the hole than the filters. This will cause the hole to grow in size over time. And as more and more air passes through the hole, more dust will bypass the filter and contaminate the cleaned air.

Pressure Gage Is Recommended

When using filters, it is always recommended to install a differential pressure gage across the filter. As dust builds up on the filter, the differential pressure will increase. And as the pressure drop across the filter increases, less air will be flowing through the filter. So when you notice a high pressure drop across the filter, this means it is time to remove and replace or clean the filters. If you don’t replace the filter and the pressure drop decreases then it probably means that you have a hole (or holes) in your filter.

So when you are cleaning your filters, be sure to check that no holes develop. The holes can be caused by high airflow when blowing out or after repeated expansions by compressed air. The use of differential pressure gages will help you identify if a hole develops before noticing dust in the clean air. If you find that you are cleaning or replacing filters too often, installing a pre-filter such as a cyclone can lower the dust loading on the filters, thereby extending their life.

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The main reason to use wet scrubbing for particulate removal is that you need wet scrubbing for chemical or fume removal. Wet scrubbers such as a packed tower are great for removing chemicals (contaminant) in the air, such as acids, bases, etc.

The Functions of Wet Scrubber

The wet scrubber sprays water droplets from nozzles which make contact with the water-soluble contaminant. Then based on the vapor pressure, the contaminant enters the water solution and leaves the air. Often times, the water has chemicals which react with the contaminant and forms a salt. This allows more of the contaminant to be collected.   In packed towers, the packing is used to break up water droplets into smaller and smaller particles, thereby increasing the water’s surface area and increasing the rate of removal of the contaminant.

The falling water droplets also come in contact with particulates. That water encloses the particulate as it makes contact, increasing the size and weight of the particulate. This makes it more likely that the combined droplet will fall to the sump and leave the airstream.

What is Blowdown?

The particulate then needs to be removed from the sump or else it can settle and eventually decrease the water in the system, which could cause operational issues. Abrasive material can wear out the instrumentation, piping, nozzles, and pumps. The wastewater created from the wet scrubber is called the blowdown. This blowdown liquid is removed from the sump either by diverting some from the recycle line or an overflow line to keep salts and particulate at a safe concentration. The particulate in the blowdown stream needs to be treated by a plant or municipal wastewater treatment plant so that it can be reused or disposed of. High amounts of solids in the blowdown stream can increase the price of treatment and cause maintenance issues on the equipment used for treatment. Many municipal plants will limit the water it allows to be treated.

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When we think about dust particles being picked up by dust collectors we think of the particles as small spheres. And often for theoretical removal efficiency of a dust collector the dust is assumed to be a sphere. This is done to simplify the calculations as spheres act predictably compared to other shapes. However in real life dust isn’t normally spherical in shape. Dust is often created by material rubbing against each other and breaking off. This material often breaks along weak points in the material structure, which often isn’t spherical in shape. The material can have a wide range of shapes.

The heavier the material the less the material shape affects how the particle acts in the airstreams. While the lighter the particle the more subjective the particle is to the forces being applied by the airstream. For example, two pieces of dust that weigh the same will act differently if the volumes of the two particles are different. The lighter unit (larger volume) will be harder to capture in a dust collector than the heavier one.

Spherical Particles

Spherical particles are easier to predict when flowing through an airstream. A spherical dust particle looks exactly the same no matter what angle you are looking at it. So basically the particle will react the same no matter which way a force acts on it. However as the particle becomes less spherical, the geometry of the particle offers more area for forces to affect it from certain angles and less from others. For example, a cylindrical particle will have smaller surface area if the force hits the round ends (let’s call these top and bottom) of the cylinder rather than hitting the longer straight walls (let’s call these the sides) of the cylinder. And the more surface area available the more force will be applied to the particle, which can cause the particle to move in the airstream, ex. spin, wobble, etc.

Particle vs. Filter

To understand how the shape of a particle can affect its removal in a dust collector let’s look at the cylindrical dust above heading to a fabric filter. If the particle reaches the filter with its side facing the filter, the chances of it getting through the small openings in the filter are very small. While if the particle reaches the filter with the top/bottom facing the filter, it could possibly slip through the opening and get past the filter. So as you see, the orientation of the particle could affect whether it is being collected or not. And since there are thousands of particles moving through the dust collector, there will be a small percentage that will hit the filter just right and pass through, thereby lowering the removal efficiency of the dust collector.

So if your dust collector isn’t getting the removal efficiency that you expected from theoretical calculations, the particle shapes could be the cause of the lower removal efficiency. Special particle size tests can be done to show the different shapes so that the particle shapes can be taken into account.

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One common mistake operators make in dust collectors is to use the dust collector hopper to store material. The hopper in a dust collector isn’t designed for this. Instead it is a temporary home for the dust collected while the material is being removed. This period should be as short as possible for the following reasons:

• The dust collector isn’t designed to support a hopper fully filled with material. The added weight could cause structural issues with the vessel and the supports.
• As the hopper gets fuller, there is a greater chance that material will be re-entrained into the airstream, thereby causing a lower removal efficiency, increased wearing on the housing, filters, etc.
• Large amounts of dust in the hopper could become airborne during an incident, which could fuel an explosion in the dust collector. Removing the material from the hopper isolates the material storage from the dust collector.
• Storage in the hopper could cause bridging or rat holing of the material. This could cause the material to backup into the separation zone (example: begin covering filters). This will cause major operational issues in the dust collector and decrease removal efficiency and airflow through the system. 

So when you are operating your dust collector, be sure to remove the dust collected in the hopper as soon as you can. Airlocks such as rotary valves, double dump valves and trickle valves (ex. Aerodyne’s Vacu-Valve) are ideal for keeping the process isolated from the outside while also allowing the collected material to leave the collection hopper.

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The airflow in dust collection systems is crucial to the proper operation of the system. The dust collection system consists of the hood/pickup points, ductwork, dust collector(s) and exhaust fan. The hood/pickup points are designed to capture the dust. This design requires a range of airflows to properly work. If too little airflow is going through the hood, dust will escape from the hood. If too much air is going through the hood, the system can capture material it isn’t supposed to (example picking up product from the conveyor belt, not just dust lingering over the belt in the air).

The Important of Ductwork

Ductwork is like a highway for the dust in the dust collection system. It allows the airflow to be directed to the dust collector from the pickup points/hoods. The ductwork should be sized so that the airflow velocity is fast enough to keep the dust in suspension, but taking into account that the faster the airflow the higher the pressure drop is through the ductwork. The minimum velocity required to keep the dust in suspension is dependent on the dust. It can vary, but usually 4500 FPM is a safe velocity. Elbows can also increase the pressure drop of ductwork so try to minimize the elbows when designing the ductwork. Make sure you know the pressure drop in your ductwork so you can have enough static pressure in your fan to keep the design airflow.

Pressure Drop in Dust Collectors

Dust collectors have a pressure drop associated with them. Usually the higher the pressure drop the greater your removal efficiency will be, however different technologies will have different pressure drop and removal efficiencies. For example 10”WC pressure drop on a cyclone will have a lower removal efficiency than 10”WC on a filter. Be sure you have enough static pressure to operate your dust collector throughout the normal life of it. A fabric filter will build up dust on it, thereby increasing its pressure drop over time. Be sure to have enough static pressure to handle the required airflow at the point the filters are dirtiest, or else your airflow will suffer.

Exhaust Fan vs. Airflow

The exhaust fan should be designed to provide the required airflow with enough static pressure throughout the operational life of the system. This means having enough static available at the higher airflow (pressure drop increased in hood, ductwork, and dust collector) and to handle dirty filters until they are replaced (or cleaned). It is often good to slightly oversize the fan and to use a variable frequency drive (VFD) to adjust the fan as required.

Another good device is a digital airflow meter (such as Aerodyne’s GPC airflow meter) which lets you monitor the airflow through the system to be sure it is operating as it was designed.

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Material of construction is extremely important to the durability of a piece of equipment. The suitability of the materials of construction is based on the process, which includes the different components, phases, temperature, and pressure. For example: a cyclone’s material of construction would be dependent on the following:

Material (dust) Being Collected

Properties of the dust being might dictate the materials of construction. Food or pharmaceuticals will usually require stainless steel to prevent / minimize contamination. Carbon steel is often acceptable for wood applications. Other times, the material properties require a special material. For example, abrasive material might require AR steel or a coating to help prevent erosion of the cyclone.

Chemical Composition

The chemical composition of the process can dictate the materials of construction. If a component will react with the materials of construction, it could cause premature failure. For example, water or high humidity can cause rusting of carbon steel so stainless steel might be better suited. Or if an acid is a component of the gas stream, then a high alloy metal or fiberglass construction might be better suited.

Temperature and Pressure

High or low temperature can cause materials to change their properties. Material can become brittle or they may react more with components. For example fiberglass can’t handle higher temperatures whereas metals usually can. And some material might have good compatibility at lower temperatures but very poor compatibility at higher ones. Sometimes pressure can affect the material properties, but usually it will affect the thickness of the walls.

Aesthetics

Some customers have plant requirements for their equipment. For example, a food or pharmaceutical facility may require stainless steel construction even though the equipment is on the waste process and everything will be disposed of.   In a similar vein, the finishing of the equipment might be determined because all the other equipment has a high polished finish and the customer wants it to fit in.

So, when determining the materials of construction, there are many factors that contribute. It is best if the end user, with help from the vendor(s), determine the materials of construction. The end user usually knows the process better than the equipment manufacturer and therefore is in the best position to determine the materials of construction.

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Rotary valves will occasionally jam. This can happen when material gets between the rotor and the housing or when oversized material cannot fit into the rotary valve pockets.   Depending on the type of jamming that is happening you handle the situation differently.

Space Between Rotor and Housing

Rotary valves have a space between the rotor and the housing. This space allows the rotors to rotate freely but it can also allow air to leak across the valve. When materials falls on the rotor edge it can buildup and jam the rotary valve. So for existing rotary valves, you might have to replace or modify the existing rotor. Beveling or chamfering the rotor edges help the material to fall off the edge and into the valve pockets.

When Jams Caused by ...

For jams caused by oversized material, a valve with larger pockets is required. This could mean a larger rotary valve or replacing the rotor with a rotor containing one less pocket. Please note if the rotary valve is on an explosive application, NFPA requires a minimum of six vanes on the rotor. If these options aren’t possible, looking into a double dump valve might provide larger clearance for oversized material to pass through.

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One of the issues when figuring out if your dust is explosive is the particle size of the dust and the location where you are assessing the dust. Material that consists of very large particles sizes (larger than 500 micron) tend not to be explosive. While larger particles can catch fire, they tend to prevent explosions. However, this might not be as straight forward as you think.

Is the Process Explosive?

Let’s assume that you have a conveyor belt that feeds a grinder which then empties onto another conveyor belt which feeds a bagging station. Let’s assume that the material starts out with a particle size distribution of 10% less than 500 micron and 90% greater than 500 micron of a combustible material. After it is fed into the grinder, its target size is around 200 microns. The question is, is this process explosive? Your 1^st thought would be that before the grinder, the dust is too large to be explosive, and you might want to take the material leaving the grinder and have it tested. So you test the material leaving the grinder on the conveyor and find out it isn’t explosive. Therefore you assume the whole process isn’t explosive. However, this might give you a false reading.

Is the Dust Cloud Explosive?

For example, the material falling on the conveyor belt that feeds the grinder creates a dust cloud. That dust cloud is made up of the finer particles, so it is probably made up mostly of the 10% that’s less than 500 micron. And this material could be explosive. Not only that, but if that 10% of the material particle size distribution has a significant amount of dust less than 100 micron, this could very well be the dust cloud which is much more likely to be explosive. And since the smaller the particle size the better the chance of the material being explosive, your dust cloud could be explosive dust. This would then float in the building and layer the surrounding equipment or rafters with explosive dust.

Now looking at the material leaving the grinder, even though it tested as not explosive this is the material on the conveyer or that was bagged. If the grinder feed creates a dust cloud then this will again be the finer dust. So while the majority of your material is around 200 micron, there is a small portion (now much greater than the initial amount that was less than 100 micron) that is very fine. And again, this portion of the dust is the dust that is creating the dust clouds at the exit of the grinder and in the bagging section.

So when you are collecting dust to see if it is explosive, be sure to collect the material that is pertinent. You should try to collect dust that is actually forming a cloud to make sure it is explosive and not lump it in with material that won’t form the cloud. If you have an existing baghouse or cartridge collector, use the material that is collected in the dust collector for testing.

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